The present invention relates to a defined kaolin composition exhibiting high opacity and to a method of producing the kaolin composition by controllably defining a kaolin clay. More specifically, the present invention relates to a beneficiated kaolin product having improved opacifying efficiency thereby rendering the product functional as a high performance paper coating or filler, and to a method of producing the beneficiated kaolin product by controllably defining and otherwise treating a base kaolin clay so as to remove a substantial portion of colloidal particles therein.
It is well known in the paper industry, that a wide variety of pigments, such as titanium dioxide, calcium carbonate, talc, synthetic silicates, and clays such as bentonite and kaolin, are suitable for use as paper fillers and/or coatings. Kaolin, a naturally occurring hydrated aluminum silicate, is presently the most widely utilized and is available in a range of particle sizes and brightnesses, as well as being either delaminated or non-delaminated. Hydrated kaolin is white in color, has a fine particle size, is relatively chemically inert, and makes an ideal low cost paper filler. Although calcined (anhydrous) kaolin is also available for use as a paper filler and can impart greater opacity to paper than the hydrated kaolin, it has the serious disadvantage of being more abrasive.
Prior art kaolin paper fillers and coatings are typically produced by a beneficiation process which typically consists of fractionating in a continuous centrifuge to remove oversize material followed by leaching to remove iron-based colored compounds. In the leaching process the kaolin is acidified with H.sub.2 SO.sub.4 to a pH of 3.0 to solubilize the iron. Sodium hydrosulfite is then added to reduce the iron to a more soluble ferrous form which is removed during the dewatering process. The flocculated clay, generally at approximately 30% solids by weight, is then filtered, such as by dewatering on a rotary vacuum filter to a solids level approximately 60% by weight. The filter cake is then either dried or redispersed with additional dry clay if it is to be sold as approximately 70% by weight solids slurry. To produce high brightness products, i.e., a product having a brightness index greater than 90, impurities may be removed from the kaolin clay by further processing the kaolin clay through flotation or magnetic separation. To produce a delaminated product, the coarse fraction from the initial centrifugation is ground in sand grinders to shear the stacks of platelets normally found in kaolin and thereby produce individual particles having an equivalent spherical diameter less than 2 microns.
It is well appreciated in the art that kaolin clay pigments must have certain rheological and optical properties to be suitable for use in paper manufacture as paper coatings or paper fillers. The kaolin clay pigment must be available as a high solids suspension typically having a clay solids content of about 50% to about 70% by weight, but still exhibiting a viscosity low enough to permit efficient and economical pumping, mixability with other filler or coating components, and application to the paper. Additionally, it is of utmost importance that the kaolin pigment exhibit certain optical properties, namely high brightness, high gloss and high opacity.
The influence of particle size distribution upon the optical properties of kaolin pigments has long been appreciated in the art. For example, in commonly assigned U.S. Pat. No. 2,992,936, Rowland discloses that a kaolin clay product having the following particle size distribution (in terms of equivalent spherical diameter, e.s.d.) will consistently show improved brightness, gloss and opacity when used as a paper coating clay:
99-100% by wt. less than 5 microns e.s.d. PA1 98-100% by wt. less than 4 microns e.s.d. PA1 88-100% by wt. less than 1.7 microns e.s.d. PA1 85-97% by wt. less than 1.5 microns e.s.d. PA1 70-84% by wt. less than 1.0 micron e.s.d. PA1 25-37% by wt. less than 0.5 micron e.s.d. PA1 10-15% by wt. less than 0.3 micron e.s.d. PA1 99% by wt. less than 5 microns e.s.d. PA1 97% by wt. less than 3 microns e.s.d. PA1 90.5% by wt. less than 2 microns e.s.d. PA1 65.5% by wt. less than 1 micron e.s.d PA1 31.5% by wt. less than 0.5 micron e.s.d. PA1 12.5% by wt. less than 0.3 micron e.s.d. PA1 5.5% by wt. less than 0.17 micron e.s.d.
Rowland further discloses producing such a controlled particle size kaolin product by first degritting a kaolin clay slurry, thence passing the degritted kaolin clay slurry at 21% solids by weight through a Sharples centrifuge at 400 cc per minute at 6300 r.p.m. and then recentrifuging the overflow effluent at the same rate and r.p.m. The final overflow effluent represented a cut taken off the fine end of the degritted clay slurry and amounted to 22% by weight of the degritted clay slurry. The degritted clay slurry remaining after removal of this 22% fine cut, i.e., the combined underflows from the two centrifugation steps, was reslurried to about 20% solids with 0.15% sodium hexametaphosphate and allowed to settle by gravity through a 1 inch slip depth. The sedimented coarse clay, which amounted to about 48% of the degritted clay slurry, was discarded leaving about 30% by weight of the original degritted kaolin clay slurry as an intermediate product to be subjected to further treatment via bleaching, filtering and drying to yield a commercial coating clay product.
In a paper entitled "Chemically Induced Kaolin Floc Structures for Improved Paper Coating", presented at the 1983 TAPPI Coating Conference, W. H. Bundy et al. disclosed an improved high bulking paper coating pigment, referred to as 1089, which comprises a chemically modified kaolin produced by the Georgia Kaolin Company, Inc. and marketed under the trade name Astra-Lite. Structures of optimum functionality are said to be derived by chemically treating a base kaolin clay having a particle size distribution wherein from about 80% to 93% by weight of the kaolin particles are less than 2 microns e.s.d. to selectively flocculate a portion of the submicron fines therein thereby aggregating a portion of these fines on the surface of larger kaolin platelets and effectively inactivating a large portion of colloidal particles. Such a chemically modified kaolin coating pigment derived from a base kaolin wherein 92% by weight particles under 2 microns is presented by Bundy et al. as having a particle size distribution as follows:
Such a chemically flocculated kaolin coating pigment may be produced, for example, as disclosed in U.S. Pat. Nos. 4,075,030; 4,076,548 or 4,078,941, by selectively flocculating a base kaolin clay with the addition of either a low molecular weight (less than 1,000,000) organic flocculent such as a polyfunctional amine, e.g., ethylene diamine or hexamethylene diamine, or long carbon chain amine, with or without citric acid and, optionally, in the presence of fine mica below 150 mesh in size.
There is disclosed in U.S. Pat. No. 4,738,726, an opacifying pigment composition suitable for use as a paper filler or coating which consists essentially of particles of hydrous kaolin clay flocculated with a controlled minor amount of a cationic polyelectrolytic flocculent, e.g., a quaternary ammonium polymer salt or a diallyl ammonium polymer salt. The base kaolin clay is selected to have a particle size distribution prior to flocculation wherein less than 35% by weight are finer the 0.3 microns, i.e., colloidal.